21 December 2009

Vail Pass, on the I-70 in Colorado, has been chosen from 22 possible locations as the site for a prototype wildlife bridge. Designers are asked to come up with a structure which can carry wildlife while being more economic than a typical highway bridge (see artist's impression, vaguely reminiscent of the CZWG / Mott MacDonald Mile End Park bridge in the UK).

As well as protecting the wildlife and bridging the ecological divide created by the highway, the bridge is also there to protect motorists. Organisers estimate the average cost of a critter-vehicle collision to be $6,617 for deer, $17,483 for elk, and $30,760 for moose, although those look a bit on the low side to me (having had a friend hospitalised for several weeks after a collision with a deer). They do reckon the annual cost to the American economy to be about US$8bn.

Go to the ARC Competition website for more information, although as yet there are no dates, rules or information on the cash prize. There's no funding in place to actually build the bridge yet, but organisers hope the competition will raise the concept's profile sufficiently to raise funding.

A project has been announced to design a second footbridge over the same dock, to be built by December 2011 in time for the London 2012 Olympics. The £7m structure forms part of a key pedestrian route between Pontoon Dock DLR station and the ExCel exhibition centre.

The promoter, London Borough of Newham, is looking for "a well-considered and appropriate piece of sustainable urban design that raises the spirit and makes the local community proud", but they note that budget and programme are paramount.

The project is likely to prove quite a challenge, with the dock being roughly 130m across, occasionally used for navigation by tall ships, and the site is very close to London City airport. Some idea of previous solutions to the problem can be gained from my post on the last competition to be held at Royal Victoria Dock.

On behalf of the local authority, Ringway Jacobs have invited expressions of interest to be lodged by 4th January 2010. Shortlisted design teams will then be invited to a series of "design workshops" in January, watched over by Chris Wise and Ed McCann of Expedition Engineering.

The winning team will be appointed by the end of January to deliver their concept design by April 2010. There's a small honoraria of £2,000 for shortlisted teams, but it's far from clear what the terms of appointment or scope of work will be, with Ringway Jacobs making clear their desire to take over the detailed design once the concept is complete. The prospective carrot is therefore somewhat smaller than for a typical bridge design competition, but this is not a typical competition, more of a competitive interview.

14 December 2009

I was in Liverpool recently, and visited the Prince's Dock Footbridge (pictured below).

Built in 2001, this was the winning design in a competition run jointly by the Prince's Dock Development Company and the Liverpool Architectural Society. The contest was won by architectural student Eduard Ross, aged 23 at the time, who teamed up with his tutor Ian Wroot and structural engineers Arup to bring the design to fruition.

It's a very unusual, assymetrical design, comprising a steel wishbone arch spanning 30m, supported at one end on curved rib members, T-shaped in cross-section, with a bowstring tie hidden behind perforated cladding panels. The ribs also support the bridge deck. It seems almost impossible to understand how it works just by looking at it, but perhaps the various design details I've included will help (all taken from a 2001 Architects Journal feature).

The arch members are 406mm tubes, with the deck spanning transversely between 168mm tubes. The 2.55m wide deck consists of aluminium planks, and the cladding is powder-coated aluminium. It all looked in good condition when I was there, perhaps it was refurbished when it was recently altered to ensure that boats could pass underneath as part of the Liverpool Canal Link scheme.

There are aspects to the design that I definitely enjoy. The overall structural form, the wishbone arch both hung from and hanging curved ribs is clearly reminiscent of some great cetacean skeleton, although I don't think Prince's Dock was historically home to Liverpool's whalers. I like the opportunities that the assymetrical geometry gives to vary the ribs, it makes the bridge a more dynamic experience, and with a directionality not always found in bridge design.

The bridge only really makes sense as a portal onto the Mersey estuary (shown left). Is the inspiration a whale's rib cage? A mediaeval ogival arch? Or perhaps something more sexual? Whatever, the bridge seems to prefer one-way traffic, with the raised wishbone also providing a clear pointer as to which way to go.

The reverse view is closed down, narrowing, darker (shown below right). It's more a tunnel than a gateway, an effect clearly accentuated by all the cladding panels, perforated or not. I find these odd - they are in no sense functional (neither sheltering against rain nor protecting against wind), but it's hard to imagine that the bridge would look better without them. I think they look pretty good, although as a structural engineer I also think it's a shame that they help to hide how the bridge actually works (by obscuring the arch's bowstring tie).

I'd love to know more about how the design came to be. I'm fairly certain that both structurally and visually, it's unique, and I guess that may have been something to do with the naivety of an architectural student unconstrained by having a structural engineer looking over his shoulder. While that approach quite often leads to poor designs, in this instance I think it has proven a success.

The initial concept has also clearly benefitted from careful detailing, and close attention to the geometrical logic of the structure. The only real oddity is the absence of some of the ribs below deck level at one end. I believe these have been cut off when the bridge was recently modified to allow boats to pass below as part of the Liverpool Canal Link. That's a shame, but the alternative of raising the entire bridge would have required more extensive and hence expensive modifications of the dock banks.

The bridge was clearly built to help facilitate development of the dockside, but from what I could see on my visit, development stalled, with a number of office and hotel buildings completed, and a couple of apartment buildings looking very isolated. One side of the dock is occupied by waste ground in the guise of a surface car park. All this despite being only a few hundred metres from the Royal Liver Building. A bridge like this needs to be the centre of the action, and there was very little of that when I visited.

10 December 2009

From the delightfully-named treehugger.com, I read about Norwegian artist Vebjørn Sand's contribution to the culture-fest surrounding the current Copenhagen climate talks.

It's a bridge spanning 15m and reportedly made from Antarctic ice: the idea being that while delegates squabble, they can look out the window and watch "Antarctica" melting. It's the artist's fourth such Ice Bridge (others having been erected in Greenland earlier this year, outside the UN building in New York in 2007, pictured above left, and in Antarctica itself in 2006, pictured right).

I think it's a great idea, very beautiful, and made all the more effective by its necessary ephemerality.

Leonardo's sketchbooks (not just the one extracted here) show that he had a sound intuitive understanding of how an arch behaves, but his proposals came two centuries before any recognisably modern theory of arch analysis was developed (Philippe de La Hire's method of 1695 being perhaps the first). The span was also ambitious to say the least: the longest arch before da Vinci's time was the 72m Trezzo sull'Adda bridge, and that was unusual, with very few arch bridges of the period exceeding 30m to 40m in span. That record wasn't broken until 1903 (Paul Séjourné's Adolphe Bridge, at 85m), and even today, the largest masonry arch bridge remains the 146m span Shanxi Danhe Bridge in China, a little over half the span proposed by da Vinci.

So Leonardo's proposal (artists impression pictured right) was more than merely ambitious, it was truly fantastical. In 1502, even to build the timber centering required for such a bridge would have been impossible, and it seems unlikely that foundations could have been created to contain the gargantuan horizontal thrust and hence prevent the Golden Horn design from collapsing.

Nonetheless, the Golden Horn bridge has an undeniable romantic appeal, much of it entangled with the myth of da Vinci's genius. For Vebjørn Sand, seeing the Leonardo sketch inspired him to try and make it a reality, to see whether renaissance imagination offered a bridge between art and technology that remains relevant today. Since his artistic project's inception, the idea of the bridge as a cross-cultural symbol has taken on added resonance (designed in Europe to be built in the Islamic middle-east, the Golden Horn design could be seen either as an ecumenical hand of friendship, or a symbol of Western ideological imperialism, perhaps).

Sand has proposed to build one new version of the Leonardo design in every continent of the world, but other than Antarctica (and the temporary ice bridge in New York), the only one completed so far is at Ås, near Oslo in Norway, built in 2001 (and pictured, left). Costing £930,000 (about £3,000 per square metre of deck) this bridge was built from laminated timber, and spans 40m across a highway. It's 109m long overall, and 2.8m wide between handrails.

It's not a scale model of Leonardo's design, but an interpretation of it. The central, and both lateral supporting arches, faithfully reproduce the lines and proportions of Leonardo's sketch, and appear to mimic his design intent, with the bridge effectively widened at its springings to permit a more slender crown to resist lateral wind loads.

The arches are made from glulam beams, milled to achieve a triangular cross-section, while the deck is of stress laminated glulam. It looks a little like a stressed ribbon structure, but it isn't really: the deck spans continuously between the intermittent steel supports.

There's some concern over how durable it will be: the timber has multiple preservation systems, including the use of sacrificial boron inserts near the foundations, but some fissuring below the wearing surface was apparent even in 2003.

Although in many ways it's a beautiful bridge, especially when lit at night, from many angles it seems to me to be visually confusing, and the pale timber does not look quite so spectacular in the day time. Nonetheless, it's a remarkable structure - there can't be many (if any) other instances where a five century old bridge design has been given new life in this way.

I suspect the Turkish scheme may come to nothing, but if it goes ahead I hope they go for a design with a little more in the way of structural integrity. I also hope that Sand does make progress with his Leonardo Bridge Project, as both the ice structures and the Ås footbridge make it clear that da Vinci's design has plenty of potential when treated as an inspirational template.

Since then, I've been waiting for the RIAI to publish the other competition entries (as they said they would), but there's still no official sign of them. So, while we wait, here at least are the top five designs, the ones that made the final shortlist (they've already appeared at architecturenow, but I've added a few pictures they didn't have, as well as some of the non-shortlisted entries).

As always, links are only given if there's more information on a firm's own website, and you can click on any image to bring up the full-size version.

Alan Baxter / AedasThis struck me as a reasonably straightforward and appealing design, so far as can be discerned from the limited images available. The structure's tilted legs owe something to the Castleford Footbridge (also designed by Alan Baxter), although here and at this angle, temporary propping would be required during construction, which would have some adverse impact on the valley floor.

The pedestrian walkway is brought close to the water rather than stuck distantly up in the air. The appeal is obvious. I guess the jury disliked the intrusion in the valley, but I think it's a reasonably elegant solution.

Arup / Heneghan Peng ArchitectsThe jury's opinion was that this bridge was geometrically complex and hence likely to be expensive. It's easy to see that it's complex from the images, with the twin main box girders deforming in a way that would be very difficult to build, and the secondary footbridge even more twisty-turny in its arrangement.

It just seems somehow a bit unnecessary, trying a bit too hard to be different for no real reason.

The competition jury took the view that this design was visually too tall and dominant, and I find it hard to argue with that.

Clearly, a lot of thought has gone into the design, including the development of a tilt-down construction sequence to minimise environmental impact in the valley (at the cost of requiring expensive temporary towers and cable supports). But the deck looks very thick away from the main arch span, and that gold colour ... ick!

Update 8th December:I've received images of one more of the entries, shown below. These again raise the question of whether height was or was not appropriate in this setting: the competition jury didn't think it was, but it's important to recognise that the amount of tree cover present in the valley limits the visual dominance of even a single-pylon cable-stay solution. As one of the Knight images also makes clear, height also allows the presence of the new Metro West light rail route to be signposted from further away.

Knight Architects / ArupUpdate 15 February 2011:I found another entry, shown below, and a load of new images for the Heneghan Peng design, some of which I've added above in the appropriate place: